1. Field of the Invention
The present invention relates generally to a method of and an apparatus for producing a semiconductor single-crystal grown by the Czochralski (CZ) method, and more particularly to a CZ semiconductor single-crystal growth system of the type having a gas rectifying tube and a position sensor for detecting the position or level of the melt surface in a crucible.
2. Description of the Prior Art
The Czochralski (CZ) method is known as a semiconductor single-crystal growth method in which a single crystal ingot is grown by pulling from the melt of a semiconductor material contained in a crucible. In an apparatus used for carrying out the CZ method, in order to control the concentration of a dopant and impurities, such as oxygen and carbon, in a growing crystal, the crucible is lifted with a change in the amount of the melt in the crucible to maintain the melt surface at a constant level during the entire crystal growth process. Especially in a CZ single-crystal growing apparatus of the type having a gas rectifying tube, the lower end of the gas rectifying tube and the melt surface must be kept in a constant positional relationship.
Accordingly, accurate detection and measurement of the position or level of the melt surface are required for achieving a precision control of the dopant and impurity concentrations in the growing crystal. In the case of the apparatus embodying the gas rectifying tube, the distance or spacing between the lower end of the gas rectifying tube and the melt surface must be detected and measured with accuracy.
FIG. 3 diagrammatically shows a position sensor associated with a conventional CZ semiconductor single-crystal growing apparatus 40 for detecting the position of the melt surface. The conventional position sensor is constructed such that a beam of light emitted from a light source 42 passes through a first view port 46 on a chamber 44 and strikes upon the surface of a melt M contained in a quartz crucible C. A reflection light reflected from the melt surface propagates through a second view port 48 and is received by a photosensitive member 50. The photosensitive member 50 detects the position of the melt surface depending on the position of the reflection light received on which position is variable with a change in position of the melt surface.
However, the position sensor of the foregoing construction cannot operate properly when a growing single-crystal or a gas rectifying tube is present in the chamber. This is because, due to the presence of an obstacle (i.e., the growing single-crystal or the gas rectifying tube) that blocks propagation of light, the light beam is prevented from reaching the melt surface or the reflection light is prevented from reaching the photosensitive member. The conventional position sensor can be used only for the measurement of the position of the melt surface.
During the crystal growth process, the gas rectifying tube undergoes thermal expansion and contraction, and the amount of thermal expansion and contraction varies in a complicated manner. As a consequence of such thermal expansion and contraction of the gas rectifying tube, the distance between the lower end of the gas rectifying tube and the melt surface varies too.
As previously mentioned, this distance should preferably be maintained constant throughout the crystal growth process, otherwise the crystal characteristics of the growing crystal, especially the concentration of dopant and impurities, such as oxygen and carbon, becomes irregular and unstable.